1. Field of the Invention
This invention generally relates to full duplex communications and, more particularly, to an improved system and method for full duplex wire-line transceiver echo cancellation.
2. Description of the Related Art
FIG. 1 is a schematic block diagram of an inverted line driver as used in a full duplex wire-line transceiver system (prior art). As noted in Wikipedia, a full-duplex (FDX), sometimes referred to as a double-duplex system, allows communication in both directions, and, unlike half-duplex, allows this to happen simultaneously. Land-line telephone networks are full-duplex, since they allow both callers to speak and be heard at the same time.
Full-duplex Ethernet connections work by making simultaneous use of two physical pairs of twisted cable, where one pair is used for receiving packets and one pair is used for sending packets (two pairs per direction for some types of Ethernet), to a directly connected device. The figure depicts the transmit (TX) pair. This effectively makes the cable itself a collision-free environment and doubles the maximum data capacity that can be supported by the connection.
Hybrid echo is generated by the public switched telephone network (PSTN) through the reflection of electrical energy by a device called a hybrid. The fundamental principle is that of impedance matching. The send signal is conventionally applied to both the telephone line and a ‘balancing network’ that is designed to have the same impedance as the line. The receive signal is derived by subtracting the two, thus canceling the send audio. Early hybrids were made with transformers configured as hybrid coils that had an extra winding that could be connected out of phase. The name ‘hybrid’ comes from these special mixed-winding transformers. An effective hybrid would have high trans-hybrid loss, which means that relatively little of the send audio would appear on the receive port.
Good echo cancellation (EC) depends upon the balancing network having a frequency-vs.-impedance characteristic that accurately matches the line. Since telephone line impedances vary depending upon many factors and the relationship is not always smooth, analog hybrids are able to achieve only a few dB of guaranteed isolation. For this reason, modern hybrids use digital signal processing (DSP) to implement an adaptive least mean squares filter that automatically detects the line's impedance across the voice frequency range and adjusts to it. These DSP systems typically use multiple digital-to-analog converters (DACs), which consume an undesirable large amount of power and may lead to current matching problems. The analog portions of these circuits typically use current mirrors to match output currents, which are not always linear over the full spectrum of operating frequencies. A current mirror is a circuit designed to copy a current through one active device by controlling the current in another active device of a circuit, keeping the output current constant regardless of loading. The current being ‘copied’ can be, and sometimes is, a varying signal current. A number of current mirror circuits are known in the art.
It would be advantageous if echo cancellation in a full-duplex transmitter could be achieved using a single DAC, and without the use of conventional current mirror circuitry.